Vocal cord vibration

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Voice is sound. Sound production is based on physics. All vocal impairments occur because of a physical change in vibration. 

In the idealized situation, sound is made when: 

  1. the back of the vocal cords leave the “breathing in” position, moving together until nearly parallel, simultaneously tension is applied to the vocal cords as they narrow the airway, 
  2. air is propelled through them, usually from below, accelerating and creating Bernoulli effect. The mucosa draws inward.
  3. viewed from above, they start opening at the top edge or lip
  4. as the upper lip opens, a lower lip starts closing
  5. the puff of air just relased travels like a wave, outward over the surface of the vocal cords.
  6. the lower lip closes
  7. the upper lip closes, and
  8. a new puff of air opens the lower lip

These steps repeat over and over, creating pulses of air – vibration. 

This is a short chapter, yet it might be the most important chapter on this site. Voice is sound. Sound production is based on physics. All vocal impairments occur because of a physical change in vibration. Any time hoarseness is not described in terms of physics; the explanation has a high probability of being incorrect.

Vocal cord vibratory cycle. Photos taken every four frames. They are nearly completely closed on the left. 4 frames later the cords are opening. 4 frames later the central membranous cord has reached maximal amplitude before they will begin to close again. 4 frames later the lower lip is closing and the original mucosal wave is traveling laterally on each vocal cord, moving the thin serous secretions with it. 4 frames later the blue arrow points to the lower lip initiating the next wave.

Vocal cord vibratory cycle. Photos taken every four frames. They are nearly completely closed on the left. 4 frames later the cords are opening. 4 frames later the central membranous cord has reached maximal amplitude before they will begin to close again. 4 frames later the lower lip is closing and the original mucosal wave is traveling laterally on each vocal cord, moving the thin serous secretions with it. 4 frames later the blue arrow points to the lower lip initiating the next wave.

The upper and lower lip concept is important for understanding and identifying pathology, especially overuse pathology. The central portion of the membranous vocal cord moves through the greatest range, so problems develop where the impact forces are the greatest.Swellings can sometimes wrap around both lips. Polyps may be on both lips, but can be only on the upper lip or the lower lip. This requires viewing a stroboscopy frame by frame at times, since a lower lip lesion may be hidden half the time.

The vocal cords oscillate quite rapidly, perhaps 100 to 200 times per second during casual speaking, with smaller cords tending toward faster oscillation. In a set of near perfect cords, at a comfortable speaking pitch, we could characterize them as:

  • being open about half the time and closed about half the time,
  • letting air out in measured puffs,
  • not leaking air during the closed phase
  • vibrating regularly, and
  • vibrating symmetrically.

This oscillation creates the sound that we hear and any single note can be visualized on an oscilloscope as a sine wave – a regular vibration and when we hear it, we hear a musical tone. We can talk about the tone in terms of frequency, often measured in Hertz or vibrations per second. Hertz is a common scientific measurement that requires the use of logarithms for calculations.

We can also use a musical scale such as the chromatic scale, com-posed of 12 equally spaced pitches, to label each tone produced (C3, C3#, D3, D3#, etc) . Each succeeding note is one semi-tone higher than the previous. I use this “semi-tone method” for documenting the voice since the visual distribution of keys on the piano separates the sounds into audibly equal intervals without delving into the complexities of logarithms.

Measuring pitch and pitch notation.

When I evaluate a patient’s pitch by ear, I match the pitch to a note on a piano. Middle C on the piano is C4 – that is the fourth octave on the piano. One octave lower is C3. The numbering changes at C so the notes in an octave can be labeled

 

C3 C3# D3 D3# E3 F3 F3# G3 G3# A3 A3# B3 C4 C4#…

 

Though octave means eight and there are eight steps in the Western diatonic scale in music, there are 12 “half-steps” in this chromatic measuring system before returning to the same note (C4 has double the number of vibrations as C3). We describe these as “equal intervals”. To our ear, the distance between C3 and C3# is the same melodic interval as the distance between C4 and C4#, as well as the distance between any other half-step. 

 

However, if one uses the Hertz scale for measurement, the distance of one semitone between C3 (130.81 Hz) and C3# (138.59 Hz) is 7.78 Hz. The distance of one semitone between C4 (261.63 Hz) and C4# (277.18 Hz) is 15.55 Hz. The Hertz scale is a nonlinear, logarithmic scale and not easily added and subtracted. Arithmetic manipulation can be done, but the simplicity of using “half-steps” far outweighs the precision of the Hertz scale for clinical diagnosis.

Summary

Visualizing sound with moving pictures

  • A change in sound will be manifest as a change in vibration on endoscopic and stroboscopic examinations.